The invention relates to a presence detection system for detecting the presence of a unit of a number of portable transmission units in a compartment of a rack of one or more racks each having a number of compartments, each portable unit having a control circuit and a transmitting or receiving circuit connected to the control circuit for the wireless transmission or reception, respectively, of a signal containing an identification number assigned to the unit and each unit having a register for storing the assigned number, a reception means for receiving a scanning signal from outside the unit and a transmission means for transmitting a response signal in response to a received scanning signal, and there being added to at least one rack a detection unit having a control circuit connected to a transmission means for transmitting the scanning signal, a reception means for receiving a response signal, a memory having a location for each portable unit for storing therein a datum which indicates the presence of the portable unit in the rack and a processing means for processing a presence datum read out of the memory by the control circuit. The portable unit may, for example, be a paging unit of a personnel paging system, a transponder, an alarm transmitter or an electronic key.
A presence detection system of this sort which is used for a personnel paging system is known from practice. In the known presence detection system, each rack has a single common transmission means for supplying the scanning signal to all the portable units in the rack and a single common reception means for receiving a response signal which is transmitted by a portable unit placed in the rack.
The known detection system is known in two embodiments having a first or second mode of operation, respectively.
According to a first mode of operation of the known detection system, a control circuit of an exchange of the paging system generates, during the time between the transmission of normal paging messages, a scanning signal for each of the portable units of the paging system in succession, which scanning signal contains the identification number of the paging unit and is supplied via the rack to the receiver in the rack. A paging unit placed in the rack compares the identification number of a received scanning signal with the assigned identification number stored in the register of the paging unit. In the event of identity, the paging unit will generate the response signal, which in this mode of operation has only a binary nature and thereby indicates the possible presence of the paging unit in a rack. Because the transmission of a scanning signal takes place serially and lasts for a relatively long time, so little time may be left over in an extensive paging system containing many paging units or in a paging system having many calls that the contents of the memory locations may lag for an undesirably long time behind the current situation. Because, in a paying system, prior to transmitting a normal (i.e. for paging the unit) paging message intended for a paging unit, the exchange first checks whether the portable unit is in a rack, that is to say whether in general a user to whom the paging unit has been assigned is not present, and because the transmission of a normal message on average lasts equally as long as the transmission of a scanning signal, unnecessary paging messages are often transmitted, or erroneously not transmitted, by the exchange in an extensive or busy paging system of this type. Because a normal paging message is in general retransmitted if no response is received by the exchange from a portable unit or from a user to whom the unit has been assigned, it may under these circumstances take an undesirably long time before normal paging messages intended for different paging units are transmitted and the paging system may itself become overloaded without its normal paging capacity being fully utilised.
In relation to the serial transmission of a scanning signal, it is pointed out that in a personnel paging system, each portable unit usually has a power saving mode in which a minimum current is supplied by a supply of the paging unit, the paging unit being activated to assume an operating mode in which the paging unit can process data from a received transmission signal if the paging unit has received a lead signal for a sufficiently long time. The time for transmitting a scanning signal may therefore be, for example, 1.25 to 6.75 s. At the same time, this method of scanning also has the drawback that all the paging units are activated regardless of the identification number of the scanning signal for every transmission of a scanning signal, as a result of which the supply of the paging units becomes exhausted earlier.
According to another mode of operation of the known system, the portable units are scanned in groups in a manner such that for every thousands digit of all the identification numbers used in the system the exchange transmits a scanning signal, that every paging unit which is in a rack and of which the thousands digit of the identification number assigned to the unit is identical to the thousands digit of a received scanning signal transmits a response signal, that the exchange scans the hundreds in a similar manner on receiving a response signal for a thousands digit, and that the exchange, on receiving a response signal for a hundred, transmits all the associated identification numbers consecutively in order to be able to receive separately a response signal from every paging unit in a rack. The total cycle time for scanning the portable units for their presence in a rack may in this case be relatively short, but increases considerably as more paging units are placed in a rack. Another drawback is that the exchange requires more time to form a scanning signal in this second mode of operation of the system than in the first mode of operation of the system.